Advertising start: add return status, use more verbose logging.

Advertising start did not return a value to indicate success/failure,
this patch adds that functionality. In addition, more verbose logging of errors from
this function are provided with the removal of the related asserts.

* Minor code cleanup

src/NimBLEAdvertising.h

@@ -77,7 +77,7 @@ public:
     void addServiceUUID(const NimBLEUUID &serviceUUID);
     void addServiceUUID(const char* serviceUUID);
     void removeServiceUUID(const NimBLEUUID &serviceUUID);
-    void start(uint32_t duration = 0, void (*advCompleteCB)(NimBLEAdvertising *pAdv) = nullptr);
+    bool start(uint32_t duration = 0, void (*advCompleteCB)(NimBLEAdvertising *pAdv) = nullptr);
     void stop();
     void setAppearance(uint16_t appearance);
     void setAdvertisementType(uint8_t adv_type);